The best. Originated as a term to describe high quality drug substances. IE, "this is good shit."

Evolved to common usage for any item of high quality. However, in typical usage, the article "the" is required to denote superiority. Without the word "the" prefacing the word "shit," a different, usually negative, meaning applies.

This joint is the shit./This is potent marijuana.
His new car is the shit./His car is excellent.

This joint is shit./This is poor quality marijuana.
His new car is shit./His new car is not desirable.